How to Fix a Missing Content - Security - Policy Header

Decloak is an AI-powered web security intelligence platform that scans a site's HTTP/TLS posture, JavaScript, and third-party scripts to produce a report anyone can read. This guide is part of Decloak's library of practical, source-backed security guidance.

Key takeaways

• Add the CSP header in your server, reverse - proxy, or framework configuration.
• Deploy a Report - Only header first to collect violation data without breaking the site.
• Refine the policy iteratively, moving from a permissive baseline to a strict enforcing header.
• Replace inline code with nonces or hashes, or use 'unsafe - inline' only when absolutely required.
• Verify the header with curl, browser dev tools, or an online scanner before considering the issue resolved.

Why does a missing Content - Security - Policy header matter?

A missing CSP header leaves the browser without guidance on which resources are allowed, making it easy for attackers to inject malicious scripts. Adding CSP blocks many cross - site scripting attacks and reduces the impact of content injection flaws.

How do I add a Content - Security - Policy header on my server?

Configure your web server, reverse - proxy, or application framework to send the header in every response. For Nginx use add_header Content - Security - Policy "...";. For Apache add Header always set Content - Security - Policy "..." in .htaccess or the VirtualHost. Frameworks such as Express can use Helmet, Django can use django - csp, and Flask can use Flask - Talisman.

Should I start with Report - Only mode?

Yes. Deploy Content - Security - Policy - Report - Only first so browsers report violations without blocking content. Example header:

Content - Security - Policy - Report - Only: default-src 'self'; script-src 'self'; style-src 'self'; img-src 'self'; font-src 'self'; connect-src 'self'; frame-ancestors 'none'; report-uri /csp-violations

Collect reports for a few days, then use the data to tighten the enforcing policy.

How can I build a baseline enforcing policy?

Start with a permissive but explicit baseline and add sources only as needed. A common starting point:

Content - Security - Policy: default-src 'self';
script-src 'self' https://cdn.example.com https://www.google-analytics.com;
style-src 'self' 'unsafe-inline' https://fonts.googleapis.com;
img-src 'self' data: https:;
font-src 'self' https://fonts.gstatic.com;
connect-src 'self' https://api.example.com https://www.google-analytics.com;
frame-ancestors 'self';
base-uri 'self';
form-action 'self';
upgrade-insecure-requests

Adjust each directive based on the violation reports you gathered.

What should I do about inline scripts and styles?

Replace inline code with nonces or hashes. Generate a unique nonce per response and add it to the CSP header, e.g., script-src 'self' 'nonce-<random>'. If replacement is impossible, limit 'unsafe-inline' to the specific directives that need it and keep it off everywhere else.

How do I verify that the CSP header is correctly deployed?

Run a simple curl command and look for the header:

curl -sI https://example.com | grep -i "content-security-policy"

Check the browser developer tools console for CSP warnings or violations. Optionally run an online scanner such as Decloak's free scan to confirm the header appears and that no CSP - related findings remain.

What next after the header is working?

Monitor the report - only logs for a week to ensure no legitimate content is blocked. Then remove the Report - Only header and keep the enforcing CSP. Periodically re - run scans, especially after adding new third - party scripts or changing the site architecture.

How does Decloak help fix CSP issues?

Decloak's free scan checks the HTTP/TLS posture layer and reports a missing CSP header as a finding. The scan also shows the current header value (if present) and provides a graded recommendation, helping you confirm that your fix resolves the issue.